It's actually pretty simple. Basicly you draw each frame a circle below each ball with his colors, but without clearing the screen between frames. That will make trails that never disapear. Then you draw small circles at random positions with the background color, so eventually the trail will be painted over by the background color. Adjusting the number and size of the random circles will make the trail disappear faster or slower. If you own Pico 8, you can find the original source code here : http://www.lexaloffle.com/bbs/?tid=3467